The present invention relates to an undercoating composition for photolithography or, more particularly, relates to a novel undercoating composition for photolithography used for forming an undercoating layer on the surface of a substrate with an object to decrease the reflecting light from the substrate surface which may cause adverse influences on the quality of patterning in the overcoating photoresist layer on the undercoating layer. In particular, the invention provides an undercoating composition from which an under-coating layer can be formed between the substrate surface and the overcoating photoresist layer without the problems of undesirable phenomena of intermixing and notching and capable of exhibiting excellent crosslinkability by heating with a large selectivity ratio to give a patterned resist layer with high fidelity to the mask pattern.
One of the difficult problems in the photolithographic patterning work is that, when the substrate surface has a high reflectivity of light as is the case with a substrate surface of aluminum, tungsten silicide and the like or when the substrate surface has a step-wise level difference to cause irregular reflection of light, the fidelity of patterning in the photoresist layer relative to the mask pattern is adversely affected by the irregular reflection of light from the substrate surface or by the influences of standing waves so that the patterned images sometimes suffer from a decrease in the dimensional accuracy of the pattern or local distortion or the so-called phenomenon of notching.
The above described problem relative to the light reflection from the substrate surface is becoming more and more serious in recent years along with the progress in the semiconductor technology in which the wavelength of the light for the photolithographic patterning is shorter and shorter than heretofore with a trend of transition to the use of short-wavelength ultraviolet light such as i-line of 365 nm wavelength, far-ultraviolet light, excimer laser beams and the like as the major current. With such a short-wavelength ultraviolet light, the troubles due to reflection of light from the substrate surface are not limited to the high-reflectivity surfaces mentioned above and stepped surfaces but are caused even from conventional substrate surface such as a silicon oxide-coated surface having no particular high-reflectivity or no level differences.
Various proposals and attempts have of course been made heretofore in order to mitigate the disadvantages such as decrease in the dimensional accuracy of patterning or the phenomenon of notching due to light reflection from the substrate surface, if not completely removed. An approach in this regard is the so-called ARC (anti-reflective coating) method to provide a reflection-preventing layer intervening as an undercoating layer between the substrate surface and the photoresist layer. Various undercoating compositions have been proposed for this purpose.
For example, Japanese Patent Kokai 59-93448 proposes an undercoating composition for forming a reflection-preventing layer containing a resinous ingredient with admixture of an ultraviolet absorbing agent. The resinous ingredient there proposed, however, is limited to a polyamic acid and polybutene sulfonic acid so that the undercoating layer formed from such a composition has low adhesion to the substrate surface sometimes to cause exfoliation or occurrence of residues or scums in the patterning procedure.
Japanese Patent Publication 3-67261 and Japanese Patent Kokai 63-138353 disclose an undercoating composition having improved adhesion to the substrate surface which comprises a resin obtained by the condensation reaction between a diphenylamine compound and a melamine compound and a highly light-absorptive compound having a light-absorption band in the same wavelength region as for the photosensitizing agent contained in the photoresist layer. Further, Japanese Patent Kokai 6-35201 proposes an undercoating composition comprising a copolymeric resin of glycidyl methacrylate and methyl methacrylate and an ultraviolet-absorbing agent, Japanese Patent Kokai 6-69124 proposes use of a poly(vinyl .alpha.-cyanoacetate) as the principal ingredient in the undercoating composition and Japanese Patent Kokai 6-75378 proposes use of a copolymeric resin of maleic anhydride and a monomer having an ethylenically unsaturated double bond in the molecule in a reflection-preventing undercoating composition.
The above described undercoating compositions in the prior art commonly have several defects because the miscibility of the resinous ingredient and the ultraviolet-absorbing agent is not always high enough so that the compounding amount of the latter ingredient in the composition is limited and, if the compounding amount thereof is too large, intermixing is caused between layers so that the reflection-preventing effect is greatly decreased and, as a consequence, the fidelity of the patterned resist layer to the mask pattern cannot be high enough not to comply with the requirement for the increasingly high degree of integration in the modern electronic technology. This problem is still more serious because the selectivity ratio, which is the ratio of the thickness reduction of the undercoating layer in an etching treatment to the thickness reduction of the resist layer in the same etching treatment, exhibited by these prior art undercoating compositions can not always be high enough while a high fidelity of patterning and good working efficiency cannot be accomplished without a high selectivity ratio.
U.S. Pat. No. 5,234,990 discloses a reflection-preventing undercoating composition for deep ultraviolet light using a polysulfone resin or a polyurea sulfone resin as the principal ingredient. A very serious problem has been found in the photolithographic patterning by using such an undercoating composition that an undesirable phenomenon of undercutting takes place in the bottom of the resist layer at the interface with the undercoating layer eventually resulting in sideway falling of the patterned resist layer.
Japanese Patent Kokai 6-118631 proposes a reflection-preventing undercoating composition comprising a phenolic resin such as novolac resins and a thermal crosslinking agent therefor such as a melamine-formaldehyde resin. As an inherency of the novolac resins, the etching resistance of the reflection-preventing layer formed from such a composition is so high that, in the dry etching process to remove the undercoating layer after patterning of the resist layer, the selectivity ratio cannot be high with a large thickness reduction of the resist layer in order to completely remove the undercoating layer. Further, the thermal crosslinkability of the melamine-formaldehyde resin as the thermal crosslinking agent is not so high that a fully crosslinked undercoating layer can be obtained only by a heat treatment at a relatively high temperature for an unduly long time resulting in low productivity against the requirement for high throughput in the manufacture of semiconductor devices.